1. Technical Field
This invention relates to telemetry of human body tissue neurostimulation. More particularly, this invention relates to current monitoring in ultrasound-powered neurostimulation.
2. Description of Related Art
Directly stimulating bioelectrically excitable tissue may be beneficial as a therapeutic tool. For example, neurostimulation may be used for restoring function in cases of neural injury or disease. Neurostimulation as used herein refers to the stimulation of electrically excitable tissues of living things. This can include, for example, the human tissues of the brain, heart, muscle, and nervous system.
Recording biological events may also be beneficial as a therapeutic tool. Tissue bioelectrical events arise from the flow of ionic currents as a result of the action of cellular ionic pumps and channels, which underlie the bioelectrical activity of neural and muscle tissues in the body. These neural and muscle tissues are associated with the function of the brain, muscles, and nervous system. The ionic currents are used for electrocardiograms, electroneurograms, and electromyograms.
Two methods of neurostimulation are the application of pulsed electrical currents directly to tissue through electrodes implanted within tissue and the indirect application of electrical currents through the body surface.
Directly applied electrical currents applied to tissue are known to affect the membranes of excitable cells, causing a depolarizing effect that can lead to a cell action event that depends on its type and biological function. The pulsing of currents is sometimes needed to prevent accommodation to current flows and to fulfill certain physiologic conditions that enables electricity to be effective. Direct application of currents may have the disadvantage of requiring invasive techniques, such as inserting probes or wires into the body.
It is also possible to apply electrical currents to the body surface where they diffuse in the volume conductivity of tissue and attenuate according to well known laws. These currents can also stimulate near-surface nerves and muscle tissues to some degree, but cannot reach deeper tissues because of high electrical losses in tissue and the rise in the needed current levels to above those that would cause electrical shock and tissue damage. It is also difficult to specifically stimulate a particular area of tissue without stimulating surrounding areas.
The strong diffusion of electrical current in tissues from surface electrodes means that specific stimulation of a given nerve or nerve fiber within a bundle is difficult. There is a tendency for electrical currents applied to the body surface to broadly stimulate in undesirable ways. Implantable electrodes overcome these problems but are invasive and suffer from the undesirable need to either run wires through the skin or work with relatively bulky implanted power systems that run on batteries or are powered by external radiofrequency (RF) powering techniques.
In general, techniques that use RF induction to power an implanted device use an inductor implanted within the body that is magnetically coupled to an external RF field. Often this inductor is coupled with a capacitor to form a resonant circuit that is more efficient in coupling to applied RF energy. These devices are relatively large and can be on the order of a centimeter in size.
High frequency currents are not known to stimulate bioelectrically excitable tissues of the nervous system of the body because they are faster than physiologic events can respond. As long as they are relatively high frequency, above several tens of kilohertz and continuing up into the megahertz region currents do not stimulate bioelectrical events or sensations of pain.
A major concern in the development of neurostimulators for implantation near nerve or muscle for therapeutic applications in the human body is the size of the implant. It is preferable that the implanted devices be small and perhaps something that could be introduced into the body through minimally invasive methods, such as syringe needle injection. This is not only for ease of insertion into tissues, but so that they produce less complications such as pressure or force against sensitive tissues as a person moves or exercises. There is also less immunological response and inflammation of tissues with small devices as it reduces their attendant risk of complications. This feature tends to encourage more widespread use in situations which are elective rather than critical.
A neurostimulation device known as a Bion™ has been described which is an example of present methods of designing implantable neurostimulation devices. It is a small cylindrical electrical device which derives its energy from an externally applied RF field. As presently designed, the size of these devices ranges from 6 mm to about 1.5 cm. These devices incorporate active LSI logic and inductive RF powering.
Some versions store energy in batteries or capacitors to deliver later upon digital command and so provide electrical pulses through integral electrodes to neural tissues. These devices are targeted for therapeutic stimulation of muscle and nerves by being implanted within body tissues and in some cases are used for pain relief, treating urinary incontinence, and can be programmed to actuate nerves and muscles in the restoration of lost function in limbs. A disadvantage of these devices is their relative complexity and large size. The large size limits their medical applicability to situations where they can be introduced by surgery or through a large trocar.
The amount of neurostimulation may not always be well know. In some examples, detection of a physiological effect is the only way of knowing whether current has been applied to a bioelectrically excitable tissue. For example, the contraction of a muscle, relief of pain, or firing of a nerve may be used as a sign that current has been applied to bioelectrically excitable tissue. By observing a physiologic response it is often not necessary to be concerned about the exact current flow induced, as long as it is within a range and there are limits to the amount of current that can flow. There are applications of neurostimulation however, such as in stimulation of the esophageal muscle for purposes of gastric reflux monitoring, where the patient may not report any sensation or overt change with effective levels of stimulation.